Humour will do a lot to save an otherwise subpar episode for me, but I do have my limits. "To Change a Changeling" tells a largely uninteresting story with unclear themes and shallow characterization, but it also has a lot of great dialogue and humorous moments. The latter does a lot to keep the episode afloat, but there's only so many issues I can forgive before they start to overwhelm the humour, and while this episode is on the right side of the line, it's teetering near the edge, and it'd need to be a lot funnier for me to fully forgive a plot this dull.
While Trixie and Starlight are travelling to the Changeling hive to visit Thorax, they're captured by an unchanged Changeling, who brings them to the hive under the impression that they're enemies. Once the misunderstanding has been cleared up, the ponies realize this Changeling is Thorax's brother Pharynx, who has been unwilling to share love or accept Thorax's pacifism. Seeing that Pharynx is making the other changelings uncomfortable, Starlight and Pharynx try to convince him to change.
Pharynx is both the episode's greatest strength and its greatest weakness. On one hand, he's consistently hilarious, offering a variety of funny responses to both the pacifism of Thorax's community and to the ponies' attempts to befriend him. Having a character bluntly reject friendship isn't exactly something new to this show, but it's still fairly amusing, and the comic timing of Pharynx's curt dialogue is solid.
The thing is, the show never gives us a reason to care whether he changes or not, and despite later upping the conflict to whether he even stays in the hive, the stakes remain low. Pharynx is consistently depicted as rude and inconsiderate, a nuisance at best who doesn't think of others in any sense aside from basic security. He's funny but never particularly likeable, and it's hard to even be invested for Thorax's sake, because we never get a sense that they have a close relationship. Pharynx never demonstrates much respect for Thorax, and even when a flashback shows us Pharynx defending Thorax from bullies, Pharynx then proceeds to harass Thorax himself. Thorax seems to care more, but aside from that unconvincing flashback, we're never given a good sense of why.
But it's hard to actually hate Pharynx, because he's mostly harmless. The worst thing he does is kidnap Starlight and Trixie, and he releases them the moment Thorax tells him to. As a result, it's hard to muster up any particular emotion towards him. The most I got was thinking that he never deserved to be exiled, but the episode is barely even about that. While that does make it a little easier to see why Thorax might still care about him, it actually serves to make him less compelling, because he ultimately feels slightly generic and shallow.
Starlight and Trixie fall into the exact same boat. Both are given some snappy dialogue, and Trixie's happy self-confidence is as amusing as always, but it's unclear what they're supposed to have learned from this experience, and because Pharynx isn't very sympathetic, their internal conflict over how to deal with him isn't very relatable. They eventually conclude that they should tell Thorax to kick Pharynx out, but while they're both very nervous about telling him, we never get enough details about the changeling brothers' relationship to really empathize. There's some inherent tension in telling someone to banish his own sibling, but that conflict lacks specificity and texture. Starlight is still kinda bland here and Trixie's boasting isn't always endearing, so this extra barrier to sympathizing with them is particularly unwelcome.
Worse still, everything Starlight and Trixie successfully accomplish only serves to make matters worse for Thorax and Pharynx. When they debate who should tell Thorax to kick him out, Pharynx overhears and leaves of his own accord, and later Starlight plans to bring out the "caring" Pharynx from Thorax's flashback by luring a dangerous creature towards the hive. The latter, in particular, comes across as something which Trixie should have known to talk her down from, given that Snips and Snails did the same thing in her first appearance, and while it feels slightly more in-character for Starlight, it's unpleasant to yet again see her putting others in danger when left unattended.
I don't even know what point Starlight and Trixie's screw up is supposed to convey. If there's a moral here about not bringing dangerous animals to a settlement, the animal in question doesn't show up long enough for that to be meaningful, and while it sets up something about giving your friends difficult advice, that plot thread is never properly resolved. The denouement suggests a moral about not writing people off as a "lost cause," because Pharynx eventually does rejoin the hive and transform, but it's not quite clear why the latter happened, because Pharynx doesn't really learn anything by the end.
In its climax, the episode shifts gears entirely and pulls out a moral that, while not entirely absent from the rest of the episode, is unconvincing due to a few dubious plot aspects. The episode is trying to say that a nonviolent community like Thorax's still needs to defend itself against threats, but we know that Thorax was inspired to be nice by the actions of ponies during the invasion of Canterlot, so it's unclear why he doesn't already know that. While we know he's gentle and until recently wasn't very assertive, it reflects badly on him that he didn't recognize ponies still had means of defending themselves, given how much pony society has inspired the way he runs his hive.
Furthermore, I'm just not sure what this episode is supposed to be telling kids. Suggesting that peaceful communities still need to defend themselves is arguably a timely message, but it also seems fairly political in nautre, and this show is targeted at people who are several years away from being able to vote. If it means to say anything about individual behaviour, then representing that by contrasting an individual with a society doesn't seem like the clearest way to communicate that, and the show already has morals about nice people still needing to stand up for themselves.
So it's a good thing that the episode is funny, because that does keep it afloat in spite of all the thematic messiness and narrative tedium. "To Change a Changeling" is saved by its good dialogue and solid comedic timing, because what's here story-wise mostly falls apart under any scrutiny, and it fails to make its characters or main conflict compelling. As a whole, it's kind of a mess, but an entertaining mess is better than a boring one any day of the week, and it's hard to say that an episode with this sharp a sense of humour is entirely worthless. If only the plot were as good as the dialogue.
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